The invention relates to a method for automatically controlling a stationary gas engine, where an engine speed control deviation is computed from a set engine speed and an actual engine speed, and a set torque is determined as a correcting variable from the speed control deviation by means of a speed controller. In addition, a set volume flow is determined as a function of the set torque to establish a mixture throttle angle and a gas throttle angle, and the set volume flow is varied to adjust the gas throttle angle by a correction factor.
Stationary gas engines are used to generate electric power. The gas engine is operated at a constant lambda value of, for example, 1.7, i.e., a lean mixture with excess air. The gas engine typically includes a gas throttle for setting the gas fraction in the gas-air mixture, a mixer for mixing the combustible gas and the air, a compressor as part of an exhaust gas turbocharger, a cooler, and at least one mixture throttle. The intake volume flow in the receiver tube before the intake valves of the gas engine is set by the mixture throttle, and thus the mixture pressure in the receiver tube is also set.
EP 1 158 149 A1 describes a stationary gas engine for driving a generator. The gas engine is controlled by using a characteristic curve to compute a set lambda as a reference output from the engine output. On the basis of the set lambda, an electronic engine control unit computes a gas quantity set value for adjusting the gas throttle. In a second embodiment, the set lambda value is computed from a mixture pressure control deviation. The mixture pressure control deviation is determined from the detected actual mixture pressure in the receiver tube and the set mixture pressure, which in turn is determined from the engine output by means of a characteristic curve. In a third embodiment, as a supplement to the second embodiment, the gas quantity set value is corrected to adjust the gas throttle as a function of the position of a compressor bypass valve and the speed control deviation. A common feature of all three embodiments is the adjustment of the gas throttle to a set lambda value. In practical operation, when a change in the power assignment is made, first a change is made in the position of the mixture throttle as the power control element. This has the effect that the intake mixture volume flow also changes. Since the position of the gas throttle initially remains constant, there is also no change in the gas volume flow. This results in a changed actual lambda. When a mixture throttle is controlled to move, for example, in the closing direction, this causes enrichment of the mixture, which results in a change in output of the gas engine. As a response to this change in output, the set lambda value, the gas quantity set value, and the position of the gas throttle are then changed. In this type of automatic control, the response time, for example, when the load changes, is critical, since intervention in the lambda control is sluggish due to the system itself.
DE 103 46 983 A1 also describes a gas engine and a method for automatically controlling the fuel mixture. In this method, in a first step, an actual pressure difference of the air mass flow in a venturi mixer is determined, and, in a second step, a set pressure difference of the air mass flow is determined from the measured actual output of the gas engine. In a third step, the actual pressure difference is then brought closer to the set pressure difference by changing the amount of gas supplied by changing the position of the gas throttle. In a fourth step, the actual gas engine output that develops is detected again, and the mixture throttle is adjusted in such a way that the set/actual deviation of the pressure difference of the air mass flow in the venturi mixture is reduced. This sequential order of operations is carried out iteratively until the set/actual deviation of the pressure difference is smaller than a limit. Since a change in the position of the mixture throttle produces a change in the output of the gas engine, the position of the gas throttle must be readjusted to compensate the change in output of the gas engine. Under certain circumstances, this can cause the correcting variables to overshoot.
A method for automatically controlling a stationary gas engine is disclosed by the nonprepublished German patent application with the official file number DE 10 2007 045 195.9, in which a gas throttle and a mixture throttle are simultaneously controlled as a function of the same actuating variable, in this case a set volume flow. The set volume flow is determined on the basis of a set torque, which is computed as a correcting variable by a speed controller from a set/actual deviation of the engine speeds. To improve the operating reliability, a torque limiter is provided, by which the set torque is limited at least as a function of a fault signal, for example, in the event of a sensor failure. In practice, however, it has been found that this method is still not optimal with respect to fluctuating fuel quality or fuel density.